An important flavor component of citrus oils is limonene. Since limonene is lipid soluble, it is often added to foods as an oil-in-water emulsion. However, limonene-containing oil-in-water emulsions are susceptible to both physical instability and oxidative degradation, leading to loss of aroma and formation of off-flavors. Proteins have been found to produce both oxidatively and physically stable emulsions containing triacylglycerols. The objective of this research was to determine if whey protein isolate (WPI) could protect limonene in oil-in-water emulsion droplets more effectively than gum arabic (GA). Limonene degradation and formation of the limonene oxidation products, limonene oxide and carvone, were less in the WPI- than GA-stabilized emulsions at both pHs 3.0 and 7.0. These data suggest that WPI was able to inhibit the oxidative deterioration of limonene in oil-in-water emulsions. The ability of WPI to decrease oxidative reactions could be due to the formation of a cationic emulsion droplet interface at pH 3.0, which can repel prooxidative metals, and/or the ability of amino acids in WPI to scavenge free radical and chelate prooxidative metals.